Using a murine model of stroke, investigators in California identified
an extracellular pathway whereby neuronal cell death results from S-nitrosylation
of matrix metalloproteinase-9 (MMP-9). Their findings appear in the August
16th issue of Science.

Led by Dr. Stuart A. Lipton, the researchers at The Burnham Institute
in La Jolla, California, observed elevated MMP activity in mouse brain
parenchyma after occlusion of the middle cerebral artery for 2 hours followed
by reperfusion.

Evidence suggesting the role of neuronal nitric oxide synthase (nNOS)
included abrogation of MMP activation after treatment with an nNOS inhibitor
and in nNOS knockout mice. MMP-9 and nNOS were found to colocalize in the
ischemic cortex.

The research team generated a recombinant, truncated proMMP-9. Treatment
with S-nitrosocysteine (SNOC), a physiological NO donor, led to the formation
of S-nitrosothiol and activation of the recombinant proMMP-9. Treatment
of cultured neurons with NO-activated MMP-9 increased apoptosis that did
not take place in the presence of inactivated proMMP-9.

Dr. Lipton's team pointed out that nitrosothiols can be short-lived
and that S-nitrosylation is reversible. Therefore, they looked further
at the chemical nature of the NO-triggered modification of MMP-9 responsible
for activation. Mass spectroscopy showed that after exposure to SNOC, a
sulfonic acid derivative of the recombinant proMMP-9 was formed.

MMP-9 from rats exposed to ischemia and reperfusion also exhibited a
sulfonic acid derivative similar to that found in vitro after NO activation
of human MMP-9.

S-nitrosylation of one cysteine residue in MMP-9 was irreversibly oxidized
to a sulfinic or sulfonic acid derivative, which led to activation of MMP-9,
the researchers found. Blocking the formation of NO prevented the production
of the sulfinic and sulfonic acid derivatives.

"S-Nitrosylation and subsequent oxidation of protein thiol in the prodomain
of MMP-9 can thus lead to enzyme activation," the authors write, subjecting
the extracellular matrix to nitrosative and oxidative stress. They suggest
that other MMPs may also be activated through this pathway.

By targeting the nitrosylation of MMPs, Dr. Lipton and his associates
hope that new treatments can be developed for stroke and neurodegenerative
disorders, such as Alzheimer's disease, HIV-associated dementia and multiple
sclerosis.